Data is stored on magnetic media such as tape by writing data in a multiplicity of linear tracks. The tracks are separated along the transverse direction of the tape and a given track runs longitudinally along the tape.
In order to increase the amount of data that can be written for a given tape width, efforts have been made to make data tracks adjacent one another. The most common method for writing is to use writers that are spaced apart by a predetermined distance. During writing, several tracks are written simultaneously, with a gap positioned between each track and the tracks adjacent thereto. Then, when writing in the opposite direction, the head steps to one side and writes additional tracks, overlapping the previous tracks by a certain amount (called “shingling”).
The reader is typically slightly smaller than the writer, is aligned therewith, and is reading one single track. This is called “write wide, read narrow.” Because the reader is narrower than the writer, the reader will tend not to read adjacent tracks in spite of the “wobble” of the tape relative to the reader as the tape moves across the head.
FIG. 1 illustrates a typical multitrack tape head 100 having a multitude of read elements 102 and write elements 104, where the read elements 102 are aligned with the write elements 104. Servo elements 106 (one shown) flank the read elements 102 and are used to sense servo tracks on the medium to keep the head 100 aligned over a data track during reading/writing.
A major drawback to this traditional “shingling” method, however, is that wobble increases the probability of overwriting adjacent data tracks during writing the reverse direction. As the track width decreases, the amount of wobble (or track mis-registration) needs to decrease proportionally. As the track width is decreasing with future generations, it is becoming more difficult to decrease the track mis-registration sufficiently to avoid overlap of readers on multiple written tracks.
One solution calls for writing adjacent data tracks in the same direction. Writers positioned adjacent each other simultaneously write multiple tracks. An advantage of this type of system is that the chance of overwriting data tracks due to wobble is eliminated for the group of simultaneously written adjacent tracks. Further, because all tracks are written simultaneously, as the tape wobbles, all tracks follow the same wobble.
Simultaneous data tracks work well if the head can precisely position its readers over each data track. However, servo tracks are typically written to the tape prior to writing any data. So, during readback, even though the head is following the servo tracks, errors occur due to wobble of the written data tracks and the inherent wobble during readback, and may even be exacerbated by a wobbly servo track. The errors can result in a particular reader reading two or more tracks simultaneously, especially where track spacing is minimal or overlapping. The resultant signal is noisy and may make extraction of the data impossible.
To resolve this problem and provide other advantages, the embodiments of the invention disclosed herein proposes providing multiple readers per data track, where the readers have less than half the width of the writers or data track width.